kuma might not be very powerful, but his ability can even send the most powerfull flying.
Can he use it to drop them into an active volcano?
can he even reach flamengo if he's on guard?
would mihawk cut him down before he could touch him?

i think in the end, no one can say who the most powerful. some ability A overpowers ability B which overpoweres C which overpowers A.

I'd say Teach, mostly because he holds the strongest position story-wise. He threatened Whitebeard and acknowledges Luffy. By the power of Shonen comics, he should be one of the final enemies and thus the strongest of the shichibukai.